Field of the Invention
The present invention relates generally to pumping systems used in the production of oil and other liquids, for example, and more particularly, to a method and system for improving the efficiency and productivity of progressing cavity and electrical submersible pumps under power disturbance conditions.
Disturbances in the electrical power to variable speed drives operating progressing cavity pumps (PCP) and electrical submersible pumps (ESP) can cause significant safety, reliability, and production problems. For example, during normal pumping operation, a PCP provides significant amounts of energy to wind up the rod string, lift fluid to the surface, and lower the casing fluid level. During a power outage condition, the pump and its associated drive system lose the ability to control the energy stored in the system. Wound-in rod string torque and fluid load on the pump can cause the pump to backspin when power to the motor is cut off. An uncontrolled backspin can reach speeds that are many times the rated speed of the system. Completely uncontrolled backspin can also create excess speed that is unsafe to personnel and or damaging to the rod string or other equipment.
Loss of control of a PCP due to power disturbances causes the pump to backspin which drains fluid from the production tubing. Backspin times can last from minutes to hours depending on the specifics of the pump application. Deep wells will generally have longer backspin times than shallow wells or wells operating with high casing fluid levels.
The actual loss of production time could be more than twice the backspin time since fluid drained from the tubing and must be pumped back to the surface. Frequent power disturbances can significantly diminish productive capacity. Completely uncontrolled backspin can also create excess speed that is unsafe to personnel and/or damaging to equipment.
There are four power disturbance conditions that can cause the pump drive to fault out and leave the pump spinning backward. The four power outage conditions are: 1) a power loss or blackout condition, in which the power may go out completely, 2) a power dip or brownout condition in which the incoming voltage may be present but at reduced voltage level, 3) a phase loss condition in which the incoming three phase voltage may be reduced to single phase, and 4) a voltage imbalance condition in which the incoming three phase voltages are out of balance.
There are several drive innovations that can be used to improve safety, reliability, and production during power disturbances. Mechanical, electrical, and hydraulic braking systems have been added to PCP drive heads to prevent injury to personnel as well as damage to the rod string or other equipment. Backspin detectors have also been used in PCP and ESP applications to prevent restarting of the pumps until all stored energy has been exhausted from the system. These protective systems must be properly set up and maintained for reliable operation. Even so, there is a finite chance that PCP drive head braking mechanisms can fail to operate correctly during a backspin. Unsafe operation can be caused by incorrect set up, improper maintenance, or system component failure, for example.
Moreover, starting into a backspinning pump can cause erratic torque that damages the PCP rod string or the ESP motor and/or pump. Restart delays have been introduced to prevent this type of starting and its associated potential for pumping system damage. PCP and ESP drive systems can include a restart timer that delays pump operation after a power outage to ensure that the pump drive does not start into a backspinning load when power is restored. Restart delays allow energy stored in rod windup and fluid levels to dissipate before restarting the pump. Unfortunately, the restart delays that are required may be up to several hours on deep wells. Frequent power outages combined with long restart delays can significantly reduce production.
The restart delay should be no longer than necessary for the motor speed and torque to have diminished to zero. Known methods of setting the restart delay can result in arbitrarily long delays, which sacrifices production, or can be result in excessively short delays, which risks damage to the pumping system.
FIG. 26 shows typical parameters for a PCP installation including power outage information, pump backspin speed, pump restart delay, pump acceleration time and power outage time. FIG. 27 shows the behavior of casing and tubing flows and levels during a power outage event. The loss in production from a single power outage event using a backspin delay timer can be significant (36.3%) for a deep well such as shown in this example.
It is accordingly the primary objective of the present invention that it provide a method and system for improving the efficiency and productivity of progressing cavity and electrical submersible pumps under power disturbance conditions.
It is another objective of the present invention that it maintain the pump drive for a progressing cavity or electrical submersible pump operating during a power disturbance, including brownout conditions.
The system of the present invention must also be of construction which is both durable and long lasting, and it should also require little or no maintenance to be provided by the user throughout its operating lifetime. In order to enhance the market appeal of the apparatus of the present invention, it should also be of inexpensive construction to thereby afford it the broadest possible market. Finally, it is also an objective that all of the aforesaid advantages and objectives be achieved without incurring any substantial relative disadvantage.